Generally, efforts to improve the encapsulation of edges of flexible sheets have taken many forms over the years. One type of flexible sheet that is of particular interest is photovoltaic cell assemblies. These photovoltaic (“PV”) cell assemblies generally start with laminates in which additional features are attached with secondary methods to form a photovoltaic device. Typically, the entire PV cell assembly is either sealed in a box or bonded to a frame. It is preferred to at least have the edges of the PV cell assembly protected from environmental conditions such as moisture or other physical conditions such as mechanical loading (e.g. wind, debris, or the like).
For PV cell assemblies that are used in PV devices that are placed on building structures (e.g. roof shingles or facades, sometimes called building integrated photovoltaics “BIPV”), it is also may be important to keep the geometric shape and dimension of the PV device in a certain range. In the case of BIPV, the PV devices can work best with existing roofing materials if they mimic these existing materials. In the case of a BIPV shingle, it would ideally be matched in thickness, ability to conform to the roof deck.
Combining these features and the traits previously mentioned, into a device presents challenge. One contemplated method of integrating these features is with over-molding the PV cell assembly with the injection molding process. This process has its own challenges in working with the PV cell systems (laminates) common to solar. It is desirable to keep the over-molded area as small as possible, so as to minimize the inactive area of the PV module. The desired small area for the inactive area presents challenges with adhesion and attachment to the laminate structure. To obtain a minimum inactive area, it is desirable to mold on both sides of the laminate for increased bond area, environmental protection and mechanical protection. However, existing injection molding principles for over-molding enforce that the laminate should be against the mold cavity such that the high pressure process does not move the laminate to an undesirable location, In this case, using traditional methods, only a single side of the laminate structure would be bonded to the over-mold material.
This difficulty is compounded further in the case of a flexible laminate mimicking a traditional flexible shingle. Injection molding on both sides of a flexible laminate is generally accomplished with location features. The laminate is firmly held with these features such that the pressure of the polymer cannot move it. The relatively high pressure of the molten polymer will work to force the laminate to one of the cavity walls. This is due to what is commonly referred to as “fountain” flow. Fountain flow describes how the polymer flow is greatest at the center of the mold cavity with little or no flow at the cavity walls. As the flow enters un-filled regions of the mold, the flow splits in a “fountain”, such that half of the flow moves towards each of the corresponding walls. At the wall, the polymer is not flowing, but freezing due to the cooler metal temperature. It is through this process that most insert molding of flexible inserts is controlled, and which results in the flexible laminate being forced to one of the cavity walls.
For this reason, it is usually desirable in processing to have the laminate insert on one side of the mold. If it is not, it will require sufficiently stiff locating pins or guides. For bond strength and laminate edge sealing, greater effectiveness could be achieved if the polymer was on both sides of the laminate.
Further compounding this problem, it is commonly desirable for the product to be flat on the back side such that it can be sealed to the roof deck as is the case with traditional shingles. This suggests that all over-molding should be on the top surface of the shingle only. This side of the shingle is generally covered with a low surface energy material, such as ETFE or glass. This low surface energy material is needed to prevent dust and moisture fouling which will block sunlight and reduce the power of the device. These same anti-fouling traits make it very difficult to bond an injection moldable material to the top surface. This also results in reduction of surface area efficiency due to adding of tie-layers and their tolerances. It would be much more desirable to obtain proper bond strength and encapsulation through utilizing surface area on the inactive side of the device.
Through use of the over-molding process on both sides of the device, improved bond strength and edge sealing can be obtained without sacrificing active PV area. The injection molding process can then be used to further improve the functionality of the device by providing a means for locating the BIPV with respect to other roofing components and encapsulation of electrical devices such that traditional connectors, wire leads, junction boxes, and diodes can be incorporated into the device as integrated components.
The problem is to develop an over-molding process that places thermoplastic on both sides of a laminated PV cell assembly, in order to seal and protect the cell edges, (does not force the laminate to the edge of the mold) and does not require the use of pins or guides to suspend the laminate in the center of the tool cavity. In some cases, it is preferred that the process produces a PV device that is flat on the back (roof side) of the device, thereby requiring an offset centering of the laminate in the mold cavity.
Although the above section has focused on PV cells and PV devices, it is contemplated that the invention described below is applicable to any flexible sheet (single or multilayer) that one may want to encapsulate.
Among the literature that may pertain to this technology include the following patent documents: EP677369A1; U.S. Pat. No. 7,462,077; U.S. Pat. No. 6,926,858; U.S. Pat. No. 6,342,176; U.S. Pat. No. 4,826,598; US Patent Publication 2002/0171169; US Patent Publication 2001/0042946; and U.S. Provisional Application Nos. 61/050,341 (filed 5 May 2008); 61/098,941 (filed 22 Sep. 2008); 61/149,451 (filed 3 Feb. 2009), and PCT Applications filed concurrently herewith Ser. No. PCT/US2009/42496 published as WO 2009/137348; Ser. No. PCT/US20 US09/42523 published as WO 2009/137353; and Ser. No. PCT/US2009/42522 published as WO 2009/137352 filed concurrently with the present application, all incorporated herein by reference for all purposes.